Method of devolatizing coal fuel



April 19, 1955 E. S. PETTYJOH N METHOD OF DEVOLATIZING COAL FUEL FiledMarch 10, 1951 i E 4; i a y 52 jive/2Z0!" 7 220? JffQ/0%ZZ hzh'whxmfiiargigw United States Patent METHOD OF DEVOLATIZING COAL FUEL ElmoreShaw Pettyjohn, Evanston, Ill., assignor to The Institute of GasTechnology, Chicago, 11]., a corporation of Illinois Application March10, 1951, Serial No. 214,887

1 Claim. (Cl. 202-26) This invention relates to methods of preparingsmokeless fuels from coal containing from about 25 per cent up to 36 to38 per cent volatile matter which, when burned, yields an excess ofobjectional smoke.

More specifically, the invention relates to a method for devolatilizingand simultaneously comminuting such coal by a method involving acombustion step and yielding a finely divided partly devolatilized hotcoal which can be burned immediately or, if desired, briquetted whilestill hot.

It is an important object of the present invention to provide methodsfor converting coal of medium or high volatile content into smokelessfuels by. the use of simple apparatus of high capacity so that suchconversion of coal can be carried out on a large scale with a minimumplant investment.

Other and further objects and features of this invention will becomeapparent from the following description and appended claim asillustrated by the accompanying drawings showing, diagrammatically andby way of example, apparatus for carrying out the method of thisinvention. More particularly:

Figure 1 is a diagrammatical representation (with parts shown inelevation and other parts shown in vertical cross section) for carryingout the method of this invention; and

Figure 2 is a fragmentary cross sectional view taken along the line 22of Figure 1.

The starting material for the method of this inven tion is medium orhigh volatile coal having a volatile content ranging from about 25% upto 36 to 38%. This coal is initially comminuted to a size not exceedingabout minus 6 to 8 mesh by any suitable conventional method, forinstance, hammer milling.

Referring now to the drawing, the coarsely granular coal may becontained in a hopper 10. From the latter, the coal moves under theinfluence of gravity through a conduit 12 into a generally horizontaltubularly enclosed screw conveyor 14 driven by motor 16. The conveyor 14feeds the coal, at any desired predetermined rate, into a verticalconduit 18 discharging into a generaly horizontally extending conduit 20between the ends of the latter. Air is caused to flow through theconduit 20 from the left to the right. This air need not be under morethan 2 or 3 lbs. per square inch pressure in excess of the pressureprevailing in a short tubular vortex chamber generally indicated at 22into which the conduit 20 discharges in a generally tangentialdirection. The amount of air admitted into the conduit 20 is controlledby adjusting a valve 24. The air in question is maintained at atemperature below the softening point of the coal being treated but issuitably held at a temperature 100 F. within said softening point. Atemperature of around 400 F. has been found to give good results. Therate of air flow through the conduit 20 is sufficiently high to suspendor entrain the coal particles and thus to carry the coal particles intothe vortex chamber 22. In general, the air flows at least at a velocityof about to ft. per second, and the ratio (by weight) of coal to airshould not exceed 10 to l.

The vortex chamber 22 is made up of a tubular wall 26 and disc-shapedend Walls 28. One of the end walls 28 is centrally apertured to permitdischarge into an axial outlet conduit 30. A second valved air conduit32 discharges generally tangentially into the vortex chamber 22. Aburner 34 may be provided in the conduit 32 for heating the airdischarged through the conduit 32 into 2,706,706 Patented Apr. 19, 1955ice the vortex chamber 22, or other suitable means may be arranged forotherwise raising the temperature of this air to the level indicatedhereinbelow.

The conduit 30 may discharge into a cyclone separator 36 or other meansfor separating gases from finely divided solids. The separated solidsare discharged through a conduit 38, while the separated hot gases maypass into a conduit 40 leading to a heat exchanger 42 for preheating,the air passing through the conduit 32 ahead of the burner 34. The gasesmay be vented from the heat exchanger 42 through a conduit 44.

The air entering the vortex chamber 22 is heated to a temperatureranging from about 800 to 1500 F. The temperature in the vortex chamber22 is maintained at from 1200 and preferably from 1400 up to 1800 to1900 F. At this temperature level, the coal particles are partlydevolatilized, and the resulting vapors burn in the presence of the air,thus maintaining the vortex chamber at the indicated temperature level.

The rate of flow of air into the vortex chamber 22 through the conduit32 is adjusted to keep the coal particles in the vortex chambersuspended and in rapid vortical movement. The air should have a velocityof at least 5 to 10 ft. per second.

In the vortex chamber 22, a ratio of coal to air ranging from 1:1 to 1:4is maintained by adjusting the flow of air into the vortex chamberthrough the conduits 20 and 32 and by adjusting the rate of feed of thescrew conveyor 14. As between the conduits 20 and 32, the ratio of airflow is regulated so as to maintain the above indicated temperaturelevel in the vortex chamber. As disclosed hereinabove, the air flowingthrough the conduit 20 is maintained below the softening point of thecoal and ordinarily at about 400 F., so that this air by itself, cannotalways serve to maintain the above indicated temperature level. Hencesufiicient air at from 800 to 1200 F. must be added through the conduit32. Further, the total amount of air added must be sufficient to supportthe combustion in the vortex chamber 22.

In the vortex chamber 22, the coal particles are comminuted, due to thesimultaneous effects of several agencies. Thus, comminution is elfectedby impact and by attrition as between the coal particles and as betweenthe coal particles and the walls of the vortex chamber, under theinfluence of the centrifugal force generated by the vortically flowinggas. The relatively finely comminuted coal particles are carried out insuspension by the gas through the central axial discharge conduit 30.The relatively coarse coal particles, on the other hand, are held bycentrifugal force in the radially outermost portion of the vortexchamber until comminuted finely enough to be carried away by thesuspending gas through the central axial discharge conduit.

At the elevated temperature prevailing in the vortex chamber 22,comminution of the coal is facilitated and carried to a greater extentas compared to normal temperatures due to the greater softness of thecoal at elevated temperatures (above 700 to 900 F.).

As the coal granules enter the vortex chamber 22, the temperature of thecoal granules is rapidly raised to a level where the volatile mattertherein is vaporized. As a result, the vapors generated Within the coalgranules contribute to the comminution by cracking the granules,transforming the granules into cellular or bubble form characterized bya thin fragile wall structure, and even by explosively disintegratingthe granules. The devolatilization of the coal particles effected at theelevated temperature of the vortex chamber is uniform with respect tothe individual coal particles as well as with respect to the wholeamount of coal entering the vortex chamber because of the finecomminution effected in the vortex chamber. As a result of this rapiddevolatilization of the coal particles, and because of the rapidadmixture of previously devolatilized coal particles with those justintroduced into the vortex chamber, the coal particles do not tend toagglomerate nor to adhere to the walls of the vortex chamber.

The residence time of the coal particles in the vortex chamber isadjusted to reduce the volatile content of the coal to from 6 to 30%.Ordinarily, this residence time may range from A to 20 seconds.

Under the conditions indicated, the coal is comminuted to a finenesssuch that 80% will pass an 80 mesh screen and 60% will pass a 325 meshscreen.

The product issuing through the discharge conduit 30 is a hot suspensionof partly devolatilized coal in combustion gas and air. The combustiongas is derived principally from the combustion of vaporized volatilecoal constituents, the coal particles themselves not being burned. Ifdesired, this hot suspension can be burned immediately in a furnace forthe purpose of generating heat. The suspension, being hot and carryingfinely divided coal in suspension, can be burned rapidly and completelyin a small space to generate intense heat, as much secondary air (ifdesired, in preheated form) being added to the suspension as may berequired for complete combustion.

For making a smokeless domestic fuel, the coal suspension is passedthrough the cyclone 36 and the hot comminuted partially devolatilizedcoal recovered through the conduit 38. At its elevated temperature, thefinely divided coal can be immediately pressed to form briquettes,without the use of any binder such as tar, although such binders can beused if desired. Due to its residual volatile content (which may be aslow as 6% the coal still possesses sufiicient adhesive properties toform briquettes when subjected to a molding pressure before it hascooled down to more normal temperatures.

By way of an example, I have treated a smoky Illinois coal containing35.5% volatile matter and passing a 6 mesh screen as follows. The coalwas fed to the vortex chamber 22 at the rate of 150 lbs. per hour. Airat about 320 F. and a pressure of 6 lbs. per square inch was caused tofiow through the conduit at the rate of 166 lbs. per hour. Air at 1380F. and a pressure of 6 lbs. per square inch was caused to flow throughthe conduit 32 at the rate of 340 lbs. per hour. The temperature at thecenter of the vortex chamber was 1700 F. The product coal contained 14.2per cent volatile matter and 80 per cent thereof passed a 200 meshscreen while 60 per cent passed a 325 mesh screen. Part of this coal wasimmediately burned in suspension, without any formation of objectionablesmoke. Another part was immediately pressed into briquettes, which werethereafter burned without formation of objectionable smoke.

In the preparation of smokless fuel from coal containing 25% or morevolatile matter, the extent of devolatilization required to eliminatesmoke formation on burning varies with the original volatile content ofthe coal. In the case of coal containing 36 to 38% volatile matter (suchas Illinois coal), devolatilization down to about to 32% is sufficient.Coal containing 25 to 30% volatile matter should be treated to reducethe volatile content down to about 16 to 18%.

The method of this invention can also be used to prepare a semi-char forblending with raw coal and subsequent devolatilizing of the resultingblend to prepare a metallurgical coke. For this purpose, the coal isdevolatilized to an extent where about 16 to 23%, and preferably about17 to 18% of the volatile matter is retained. In the blend, thesemi-char should range from about 40 to This blend is subjected to aconventional high temperature carbonization yielding a high grademetallurgical coke.

Many details of procedure may be varied without departure from theprinciples of this invention and it is therefore not my purpose to limitthe patent granted on this invention otherwise than necessitated by thescope oi the appended claim.

I claim as my invention:

In a continuous process for preparing smokeless fuel in briquette formfrom a smoky coal containing 25 to 38 percent original volatile matterby simultaneously comminuting the coal and reducing the volatile matterto about 24 to 84 percent of the original volatile matter content, andforming the hot comminnted partially devolatilized coal into briquettes,the devolatilizing steps which comprise suspending the smoky coal in theform of about minus 6 mesh granules in a current of hot air at atemperature about below the softening point of said coal, continuouslyforming the suspension into a vortex and incorporating additional air atat least 800 F. to increase the ratio of coal to air to a range of from1:1 to 1:4, maintaining the temperature in said vortex at from 1200 to1900 F. to drive oif volatile matter and comminute the coal, theelevated temperature in said vortex being maintained solely bycombustion of the driven-off volatile matter in the presence of saidadditional air without burning the coal, and continuously withdrawingfinely-divided partially-devolatilized coal from the center of saidvortex after a residence time of from A1 to 20 seconds.

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